Poxviruses are a family of large DNA viruses that encode up to 200 distinct open reading frames. The large size of the poxvirus genome is an important feature that has allowed them to acquire multiple immunomodulatory genes and thereby evolve unique strategies for evasion from host anti-viral responses. Ectromelia virus (EV) is a member of the orthopoxvirus family and is a highly virulent rodent pathogen that causes the disease mousepox. EV is similar to variola virus, the causative agent of human smallpox. Our primary hypothesis is that secreted and cell membrane associated proteins encoded by EV likely serve important roles in viral evasion of host mediated innate and adaptive immune responses. Using a bioinformatics approach coupled to the established literature, we have selected 28 target proteins from the EV Moscow strain genome that will be investigated by a combination of biochemical, functional, and crystallographic tools in a high-throughput, structural genomics style approach. Our primary targets of investigation include the seven known cytokine and chemokine decoy receptors encoded by the virus that are specific for TNF, CD30L, IL-18, IFN-alpha, IFN-gamma, IL-1beta, and CC-chemokines. We are also targeting three proteins with sequence similarity to natural killer receptors of the C-type lectin family. We have the following specific aims for the exploration of these potential agents of immune subterfuge: (1) Establish baculovirus and bacterial oxidative refolding expression systems for targeted EV encoded proteins to be used in functional and structural studies; (2) Identify and characterize the interactions between EV proteins and their host ligands and receptors; (3) Determine the structural basis of EV protein function by x-ray crystallography and structure-based mutagenesis.